So-called multi-voltage electrical systems for motor vehicles are known in principle. Multi-voltage electrical systems are used, for example, when consumers in a motor vehicle have different performance requirements. Multi-voltage electrical systems, as the term is used in this application, include so-called subsystems which are configured to operate at the same or at different voltage levels, referred to in this case as “operating voltages.” In particular, multi-voltage electrical systems may be designed as dual voltage electrical systems, in which the operating voltages may total 48 V (in a so-called high voltage subsystem) and 12 V (in a so-called low voltage subsystem).
Two subsystems of a multi-voltage electrical system may be connected to each other via a DC/DC converter. At least one of the two subsystems includes a generator operable electric machine which feeds the respective subsystem. The respective other subsystem connected via the DC/DC converter may in turn be fed from the subsystem having the generator operable electric machine if the other subsystem itself does not include a generator operable electric machine.
In principle, the present invention may be used for all multi-voltage electrical systems for motor vehicles which have a first subsystem operable at a first operating voltage and a second subsystem operable at a second operating voltage. The operating voltages may be identical or different. Identical operating voltages in two subsystems are used, for example, when in one of the subsystems safety-related electrical consumers are grouped which are intended to be protected from potential voltage spikes or voltage drops in the respective other subsystem. Thus, the use of the present invention is not limited to dual-voltage electrical systems, i.e., electrical systems with exactly two subsystems. Multi-voltage electrical systems, however, include at least two subsystems referred to within the scope of this application as “first subsystem” and “second subsystem.” In conventional dual-voltage electrical systems, for example, the first subsystem has a higher operating voltage (high voltage subsystem) and the second subsystem has a lower operating voltage (low voltage subsystem).
However, the present invention relates in particular to the dual-voltage electrical system explained, in which a generator operable electric machine is provided in only one of the (two) subsystems. In this connection, the subsystem (of a dual- or multi-voltage electrical system), as the term is used in this application, which includes the generator operable electric machine, is referred to as the first subsystem. The second subsystem is then fed from the first subsystem via the DC/DC converter. Conventional DC/DC converters are typically installed as separate devices having a separate housing or as separate devices in a housing together with a pulse controlled inverter or a battery. A corresponding DC/DC converter, as mentioned, has the task of ensuring the exchange of energy between the subsystems.
Multi-voltage electrical systems are used, in particular, in so-called recuperation systems for recovering brake energy. For the purpose of recuperation, at least one generator operable electric machine is integrated into the first subsystem and designed to be able to provide sufficient braking power. The electrical system must therefore be designed as a multi-voltage electrical system. In this connection, it is known from JP 2007-259511 A1, U.S. Pat. No. 7,407,025, EP 1 219 493 B1, JP 2012-021687 A, and EP 1 138 539 B1 to use a DC/DC converter for stabilizing the voltage supply of the second subsystem.
An attenuator is typically provided in the first subsystem, for example, in the high voltage subsystem explained, which includes the generator operable electric machine and which is configured to supply the second or additional subsystems via the DC/DC converter. A so-called high voltage battery, for example, is installed in a high voltage subsystem. The attenuator may also be a capacitor (in particular a so-called super capacitor).
Typically, consumers situated in the first subsystem are coupled to the first subsystem and may be decoupled from it. The terms “couple” and “decouple” include all measures which in each case cause a current to flow into the respective consumers or a corresponding current to be suppressed, for example a switching on and off. The consumers in the first subsystem are naturally those which have correspondingly high performance requirements. If an attenuator is present in the first subsystem, sudden load variations caused by coupling or decoupling of corresponding consumers are then sufficiently compensated for by the attenuator. This means that coupling or decoupling of corresponding consumers causes no sudden load variation, which manifests itself in the form of a sudden rise in voltage or sudden drop in voltage. A corresponding voltage may fall or rise, but this occurs within a time window in which the generator operable electric machine has sufficient time to compensate for the load increase or load reduction by increasing or reducing its power output.
Problems arise, however, when a corresponding attenuator in the first subsystem fails and/or must be switched off. In such cases, it is possible to maintain the energy supply of the motor vehicle only with difficulty, because sudden load variations caused by the coupling or decoupling of the consumers may result in strong voltage fluctuations. If the generator in the first subsystem is not able to adjust corresponding sudden load variations rapidly enough, undervoltages or overvoltages may occur.
Hence, the first subsystem and, potentially, the second subsystem (through degradation of the interconnected DC/DC converter) may acquire an undervoltage when a consumer is coupled and an overvoltage when a consumer is decoupled. The result of this may be, for example, that a touch voltage limit cannot be adhered to or a provided overvoltage protective circuit is overloaded. Depending on the provided regulating strategy, voltage fluctuations may lead to consumer failures within the first subsystem or to failure of the entire first subsystem.
Moreover, switching the generator to the de-energized state (i.e., with no corresponding energy store in the first subsystem) is not readily possible, particularly if the control panel and/or the controller is/are supplied from the first subsystem. Therefore, a corresponding system in such cases is normally switched to a safe state which, however, includes switching off the first subsystem, and the vehicle malfunctions.